Perhaps the most common video display device in use today is the cathode-ray tube ("CRT"), the most common application, of course, being the household television. In recent years, the use of CRTs has been expanded to include the display of computer processed information, data recording, scan conversion, projection devices, and a myriad of other applications, including video arcade games.
The majority of cathode-ray tubes are of conventional design, differing only in size and shape of envelope, and type of electron gun. The basic and most common CRT includes a glass faceplate (or screen), which is the viewed portion of the device, with a conically shaped envelope attached to the back of the faceplate. At the small (rearmost or neck) end of the envelope is located an electron gun, which inclues a heated wire (or filament) which is capable of emitting electrons by thermionic emission. The inside of the facepate is coated with a thin layer of material which phosphoresces (emits light) when struck by energetic electrons emitted by the electron gun. The inside of the envelope is evacuated to a hard vacuum to allow the free passage of electrons from the gun to the faceplate.
The function of the electron gun is to form the electrons emitted by the filament (or cathode), into a tight beam only a few thousandths of an inch in diameter. The gun accomplishes this by way of an array of focusing grids and rings. The electrons are then accelerated toward the faceplate by means of an electric field caused by applying high voltage to a metallized coating deposited on the inside of the envelope, near the front (faceplate) of the tube. The voltage required varies from a minimum 700 to 1000 volts to as much as 80,000 volts, depending on the size of the tube, and the brightness required of the display.
The parts of a cathode-ray tube described above are capable, by themselves, of illuminating a tiny spot on the face of the CRT. In order to create a useful display, a means of directing or aiming the electron beam at specific points on the screen is added. One common method of altering the motion of electrons is by a magnetic field. In an electromagnetically deflected tube, the beam is deflected by magnetic fields caused by passing current through relatively high inductance yoke coils, usually placed around the outside of the neck of the tube. Although this method can be made to operate from low voltage power supplies, the currents required by the yoke coils increase the amount of power required by the system. The inductance of the yoke coils also places an upper limit on the frequencies which can be used to drive them. The electromagnetically deflected tube offers the advantages of good focus and can be driven by low voltage deflection circuitry. The preferred embodiment of the present invention utilizes an electromagnetic deflection system, but is readily adaptable to other deflection systems as well, e.g. electrostatic deflection systems.
CRTs are driven by different display formats. A "raster scan" display is created by rapidly sweeping electron beam horizontally across the face of the CRT, while simultaneously sweeping the beam vertically at a slower rate of speed, the arrangement used for television broadcasting. As the beam is swept, the electron beam current is modulated by the picture (video) information which causes the phosphor at different areas on the CRT face to glow at contrasting brightness levels, thus, "painting" the image on the face of the tube.
In the case of television, the horizontal scan rate is approximately 15,750 Hertz, and the vertical rate is 60 Hz. During each 1/60th of a second vertical sweep period, one frame of 262.5 lines is drawn on the screen. The scan lines of alternate frames are interlaced, thus producing a display with 525 lines of vertical resolution, 30 times per second. At this rate of speed, the human eye does not perceive a significant flicker.
Computer generated, raster scanned video displays are usually created by representing each displayed point on the screen by one bit in a computer's main memory. This type of display generation is termed "memory mapped." Since the memory requirements to display an image of the desired resolution is fixed, the amount of memory consumed by the display is the same, regardless of the complexity of the image. The data stored in the area of memory set aside for the display is read out by appropriate circuitry and used to drive the display directly. The data is read from memory, in synchrony with the electron beam scanning of the CRT. The electron beam current in the CRT is turned on or off by the presence or lack of a "true" bit at each appropriate location of the display memory, causing the appropriate points on the face of the tube to be light or dark. In order to change the displayed image, the undesired, existing points are erased, and the new desired points are added to the displayed image in memory. The requirement for erasing the existing points on the screen is a decided disadvantage to the memory mapped raster scanned video display generation method for the creation of complex or dynamically changing shapes.
Most computer displays are of the raster scan type; however, these displays are, for the most part, used for displaying of character images. Character images can be stored in low-cost read-only memory, and if these pre-stored alphanumeric characters are the only images to be displayed, the memory and processing requirements for the display can be considerably reduced. For example, this type of principle is used in one commercial apparatus in use today, the "Video Computer System", manufactured by Atari, Inc. which adapts a home television receiver to play arcade video games. A primary disadvantage of this application of the raster scan format is that the video refresh requirements are such that, when only one CPU is employed, the only available CPU time for performing game logic functions is during the vertical retrace of the CRT beam. During the horizontal scanning, the CPU is employed in refreshing the video display. The limited CPU time for game logic calculation is a significant restriction on the complexity of the games. Displays which require very high resolution, or compatibility with home television devices are also, for various reasons, implemented with raster scan video generation techniques.
A vector CRT display is entirely different from a raster scan display in that the motion of the electron beam is directly controlled by the associated electronics. In the raster scan display, only the intensity of the beam (brightness) is controlled, while the motion of the beam repeatedly traces a fixed raster pattern on the face of the tube. A vector display has direct control over the left/right and up/down motions of the beam, as well as the brightness of the beam at each point.
In a vector display system, objects are represented on the screen by a sequence of line segments, each of any length or orientation. Each line segment is defined in the display's refresh memory by its endpoints only. The X-Y coordinates of the starting point of a line segment, and the X-Y coordinates of the ending point can be specified in only four words of the memory. The intervening "points" are filled in automatically by the vector generation electronics. Furthermore, most objects are drawn with sequences of connected line segments which allow the endpoint of each line segment to double as the starting point of another line segment, thus, saving still more memory. For example, a closed quadrilateral of any shape, orientation, or dimension may be drawn with only 10 words of data.
The vector display method, then, is ideally suited to the drawing of figures and other graphic displays. The raster scan system is more suited to the display of pictures and other solid, or "colored-in" scenes. A vector display is more efficient at using available system resources in that less processing time is required for the computer to create and move shapes, less memory is required to represent those shapes, and less power is consumed by the CRT driving electronics.
Scaling, translation, or rotation of an object is also, much more easily accomplished in a vector display system, in accordance with the present invention, than in a raster system. For example, to take a line which is 100 points long and move it diagonally in a raster system requires that each of the 100 points be individually decoded from memory, translated to their appropriate destinations, written into memory, and erased from their old locations. This requires thousands of computer instructions. In contrast, moving the same line on a vector display device requires the translation of either 2 or 4 bytes of data (depending on the type of motion required), yielding an efficiency improvement of approximately 100 to 1. No erasure of old data is required, only modification of existing refresh data.
One purpose to which CRTs have been put as display devices is that of arcade video games. There has been in recent years a boom in the popularity of these games, resulting from the advent of microprocessors and microcomputers. These arcade games have developed from the relatively simple black and white video "Pong" games of a few years ago to the sophisticated games available today utilizing color displays and realistic sound effects, and capable of displaying numerous objects in a highly complex field pattern. A major drawback of these sophisticated systems is, as might be expected, their cost as well as their relatively large size. The self-contained video arcade apparatus of today cost in the thousands of dollars, and are large freestanding pieces of equipment generally adapted to a professional arcade use.
Apparatus have been developed to simulate the arcade game experience in the home by utilizing apparatus which relies on the home television receiver as the display means. As mentioned above, such apparatus is manufactured by Atari (the "Atari Video Computer") and, as well as other companies, e.g., Mattel, Inc. (the "Intellevision" game) and Magnavox (the "Oddessy System"). Apparatus which relies on the home television receiver must be equipped to generate signals compatible with the receiver, i.e., RF frequency carrier signals modulated in the same manner as broadcast signals to allow demodulation and display by the relatively high resolution raster scan display process of the cathode-ray tube receiver. The added cost and complexities of circuitry for modulating the data signal to RF frequencies is a considerable drawback to such systems. Yet another drawback to the home system which utilizes a television receiver as the display device is the fact that the game apparatus must be adapted to generate its display using the raster scan technique employed in the standard television receiver. This technique may be well suited to the television receiver application, since it is capable of relatively high resolution display required for acceptable reproductions of motion picture, live events or other works recorded by photographic or video methods.
Certain professional grade arcade video games utilize a vector display format instead of the raster scan format. Typical of the publications in this art are U.S. Pat. Nos. 4,027,148 (for a vector generator) and 4,053,740 (for a video game system), both issued to Lawrence T. Rosenthal. The video system and vector generation techniques described in these publications are believed to be used in present day professional quality video arcade games, and are adapted to generate separate line segments which, when viewed as a composite, appear as recognizable animated objects. Yet the Rosethal disclosures still fall considerably short of providing a system which takes full advantage of the advantages of the vector scan format. For example, Rosenthal's technique of drawing individual line segments fails to recognize that arcade games may be viewed as object oriented, and not simply as an accumulation of line segments. The Rosenthal system is unable to provide efficient control of the intensity of individual line segments, to efficiently scale object size, and, moreover, is oriented to a professional, expensive grade of equipment.
The Rosenthal vector generation system suffers several disadvantages. A principle disadvantage is that the system employs a separate digital-to-analog converter ("DAC") for the X coordinate and Y coordinate components of the vectors. DACs are relatively expensive circuit components, and moreover can require adjustments to compensate for shifts in component values due to aging, etc. Such adjustments required costly service calls by trained technicians. Failure to maintain proper alignment will shift line segments and affect the quality of display, as line segments defining an object may fail to connect, due misalignment offsets. The Rosenthal vector generation system suffers the disadvantage that absolute drawing voltages for each line segment are generated; the system does not efficiently use differential voltage levels to draw differential line segments, or to efficiently utilize voltage levels generated to draw the preceeding vector.
These and other shortcomings of the prior art known to applicant have been resolved by the preferred embodiment disclosed herein, which includes a self-contained video display and game apparatus, adapted for the display of objects consisting of connected line segments. A dedicated CRT is controlled by a microprocessor unit which is adapted, with associated peripheral circuitry, to convert digital representations of objects into a series of connected line segments drawn by the CRT. The digital representation includes sets of data defining the line segments representing the object. The controller is adapted to determine the position of the object to be drawn, to draw a (blanked) line segment from the origin or center of the CRT to that location, and to draw the series of predetermined, connected line segments which comprise the object. The apparatus is further capable of varying the intensity of each line segment from zero to the maximum intensity and to vary the size of a displayed object by changing the writing time of each line segment.
The apparatus is provided with means for receiving an external ROM cartridge, storing additional game and object information. The system is also readily adaptable to color CRT displays.
The CRT beam control apparatus of the preferred embodiment includes a single digital-to-analog converter, three sample-and-hold circuits, and a pair of active integrator circuits. A microprocessor is adapted to control these circuit components and to cause digital representations of the differential X and Y components of line segments and the intensity level to be loaded into the DAC. A data multiplexer selectively applies the DAC output to one of the preselected sample-and-hold circuit means.
In the preferred embodiment, the microprocessor is operative to provide a digital word to the DAC representative of the differential X component of the line segment to be drawn. The data multiplexer applies the analog signal output of the DAC to a first sample-and-hold circuit. A digital word representative of the intensity level is also applied to the DACs and the analog output applied to a second sample-and-hold circuit. A digital word representative of the differential Y component of the line segment to be drawn is loaded into the DAC and the analog signal coupled directly to one of two integrator circuits (the X integrator) via a first switch means. The output of the first sample-and-hold circuit is coupled to the X integrator circuit via second switch means. The control gates of the first and second switches are ganged together.
After the DAC has converted the X and Y differential component values, the two switch means are closed for a predetermined interval, applying the X and Y analog voltage signals to the respective X and Y integrators. The integrator is operative in the conventional manner to integrate the applied voltage over time. Hence, with a constant voltage input, the integrator output will linearly change with time, over the interval during which the switches remain closed. The integrator outputs are applied to X and Y deflection circuitry means and comprise the drawing signals which operate to deflect the electron beam to draw a line segment.
The integrators are operative to selectively hold the voltage level at the end of a first drawing interval so that application of successive X and Y differential voltages is added to (or subtracted from) the initial output level of the integrator. These features provide a significant advantage, allowing a plurality of connected line segments to be drawn from differential signal levels, rather than from absolute signal levels referenced from a particular origin location. Means are provided for selectively zeroing the integrators.
Yet another feature of the present invention is the multiple use of the single DAC. The DAC is used to convert data representative of the preselected intensity level.
The problem of DAC adjustment is substantially overcome by use of the DAC "00" output as the virtual ground provided to the differential amplifiers which drive the X and Y integrators. By periodically setting the DAC to the 00 level and selectively applying that DAC output to the third sample-and-hold circuit and then to the "plus" input of each differential amplifier, the integrators will shift with any changes in the DAC. This compensation technique substantially eliminates the need for periodic maintenance of the DAC.
The disclosed apparatus has an ability to control a wide range of the characteristics of a CRT not heretofore available in a relatively low cost system. The degree of control over the beam allows the use of the CRT not only in a vector scan format, but also in a pseudo-raster scan format.
Other features and advantages of the preferred embodiment are described hereinafter.